1. Field of the Invention
The present invention relates to two-dimensional autofocus methods in a Synthetic Aperture Radar (“SAR”) systems. More specifically, the present invention relates to two-dimensional autofocus methods which compensate both on and off-axis distortions in SAR signal history.
2. Discussion of Background Information
Synthetic aperture radar (“SAR”) systems typically involve a single radar antenna mounted on a moving platform. Data is generally collected using an antenna having a relatively small aperture. Because the antenna is mounted on a moving platform, a series of recurring radar pulse returns from the same target may be received by the antenna at different spatial locations. These returns may then be synthesized during processing, thereby simulating the use of a much larger antenna aperture. As such, it is possible to create an image having a higher azimuth resolution than would be possible with the same antenna at a stationary position.
It is well known that phase errors often cause a SAR image to be out of focus. These phase errors may be the result of uncompensated motion between the SAR antenna and the scene being imaged, algorithm approximations, hardware limitations or propagation effects. Therefore, to improve image quality, it is desirable to remove, compensate for or correct for these phase errors.
Generally, one or more autofocus algorithms are used to remove or correct for these phase errors. The algorithms are typically computer-implemented estimations that are applied to the SAR signal history to remove or compensate for any degradation of the received signals. Previous systems have utilized autofocus methods such as mapdrift, pulse pair product (including phase gradient autofocus (“PGA”), shear PGA, eigenvector phase history and phase difference autofocus), contrast optimization and prominent point processing.
However, these previous methods are only capable of removing one-dimensional phase error. That is, only phase errors which appear as distortions of a target in the cardinal (i.e., range and azimuth) directions may be removed. While one-dimensional autofocus is sufficient for mid-resolution SAR systems where the dwell angle is relatively small, as the resolution of a SAR system increases, non-linear target distortions become more significant due to an increased dwell angle.